Sum of the points ot the two rolls
Roll 1 1 2 3 4 5 6
Roll 2
1 2 3 4 5 6 7
2 3 4 5 6 7 8
3 4 5 6 7 8 9
4 5 6 7 8 9 10
5 6 7 8 9 10 11
6 7 8 9 10 11 12
Prime results:
2: once, 1
3: twice, 2
5: four times, 4
7: six times, 6
11: twice, 2
The easiest way is to count all the results in which none of the rolls was a three and the sum of the two rolls was not a prime number: it is 14 results.
Now subtract those 14 results from the 36 possible outcomes, the difference is the number of results in which either the sum of the rolls is a prime or at least one of the numbers is three: 36 – 14 = 22
Probability = positive events / total events
Probability: 22/36 = 11/18
Please, let me know if my explanation was clear enough.